Accumulation of mitochondrially synthesized Saccharomyces cerevisiae Cox2p and Cox3p depends on targeting information in untranslated portions of their mRNAs.

The essential products of the yeast mitochondrial translation system are seven hydrophobic membrane proteins and Var1p, a hydrophilic protein in the small ribosomal subunit. Translation of the membrane proteins depends on nuclearly encoded, mRNA-specific translational activators that recognize the 5'-untranslated leaders of their target mRNAs. These translational activators are themselves membrane associated and could therefore tether translation to the inner membrane. In this study, we tested whether chimeric mRNAs with the untranslated sequences normally present on the mRNA encoding soluble Var1p, can direct functional expression of coding sequences specifying the integral membrane proteins Cox2p and Cox3p. DNA sequences specifying these chimeric mRNAs were inserted into mtDNA at the VAR1 locus and expressed in strains containing a nuclearly localized plasmid that supplies a functional form of Var1p, imported from the cytoplasm. Although cells expressing these chimeric mRNAs actively synthesized both membrane proteins, they were severely deficient in cytochrome c oxidase activity and in the accumulation of Cox2p and Cox3p, respectively. These data strongly support the physiological importance of interactions between membrane-bound mRNA-specific translational activators and the native 5'-untranslated leaders of the COX2 and COX3 mRNAs for localizing productive synthesis of Cox2p and Cox3p to the inner membrane.